In response to health concerns relating to the accumulation of toxic, noxious and/or other undesirable fumes or gases in the occupancy areas of residential and other buildings, various air removal or exhaust systems are commonly used. Prior exhaust systems have generally included an exhaust fan and associated duct work for exhausting the air to the outside of the building. The exhaust fans for conventional air removal systems have been controlled either manually or by means of a programmable timer which actuates the fan periodically or for preselected time periods. Manually controlled exhaust systems are generally unreliable, ,especially for removing undesirable air contaminants which are imperceptible to the human senses. Timer-controlled exhaust systems are more reliable, but are still unresponsive to the actual concentration of an undesirable contaminant. The concentration of a contaminant in an occupancy area of a building is generally dependent on a variety of unpredictable factors such as air infiltration rates, heating, air conditioner and ventilation operation, and the extent to which windows and doors have been open. Accordingly, timer-controlled exhaust systems are not well suited for maintaining the level of undesirable contaminants below a prescribed level because they are either operated for a shorter time period than necessary, resulting in a possible health risk, or operated for a longer time than is necessary, which can result in a significant waste of energy due to the escape of heated or cooled air and unnecessary fan operation.